In a Communication network such as Global System for Mobile Communications (GSM, often called 2G), Universal Mobile Terrestrial System (UMTS, often called 3G), LTE (Long Term Evolution) network, WLAN (Wireless Local Access Network) networks or HRPD (High Rate Packet Data) networks, AAA servers may provide functionality of Authentication, Authorization and Accounting to both operators and subscribers. FIG. 1 illustrates a network architecture comprising one or more AAA servers. A UE (User Equipment) may be connected to a packet data communication network via the PND-GW (Packet Data Network-Gateway). The UE using a 3GPP (3rd Generation Partnership Project) access network is connected to the PDN-GW via the so-called SGW (Serving GateWay). The UE using a so-called untrusted non-3GPP access network, e.g. WLAN, is connected to the PDN-GW via the ePDG (evolved Packet Data Network-Gateway). The UE using a so-called trusted non-3GPP access network, e.g. HRPD or WLAN, is connected to the PDN-GW via an access gateway (not shown in the figure) inside the trusted non-3GPP access network. The UE's data connections are shown by solid lines and the corresponding radio connections with double-dotted lines in FIG. 1.
The network entities described above are also connected to the AAA Server (or Proxy AAA Server) in order to request authentication of the UE. The AAA Server in turn may be connected to a HSS (Home Subscriber Server) in order to request authentication information for the UE or to request subscriber information associated with the UE. The signaling connections to and from the AAA Server are shown by solid lines in FIG. 1.
In the case of roaming situation (i.e. when a UE has left its home network and is residing in a visited network), PND-GW, ePDG, untrusted non-3GPP access network and Trusted non-3GPP access network may be connected to the AAA servers via a proxy AAA server as shown by the dashed lines in FIG. 1.
In reality, an AAA server (e.g. a first AAA server 203) may fail, either temporarily or for a longer period, as shown in FIG. 2. In order to minimize the impact of the failure, any affected network entity 201 may resort to the service from an alternative AAA server (e.g. a second AAA server 204). The affected network entity 201 may be any of a PND-GW, an ePDG, a network element within an untrusted non-3GPP access network or a network element within a trusted non-3GPP access network in non-roaming situation or a proxy AAA server in roaming situation.
FIG. 3 further depicts the details of the failure situation. Assuming an AAA server, e.g. the first AAA server 203, suddenly fails as shown in 301. An access indication or authentication request 302 comprising a user identity, e.g. IMSI (International Mobile Subscriber Identity), associated with an UE (not shown in FIG. 2) arrives at the network entity 201. Said network entity 201 may send a corresponding authentication request 303 comprising the user identity to an AAA server (e.g. the first AAA server 203), which is associated with the user identity, as it is not aware that the first AAA server 203 has failed. Consequently, no response will be received from the first AAA server 203, which may, e.g. trigger a timer 304. When the timer expires, the network entity 201 may select an alternative AAA server, e.g. the second AAA server 204, and re-send the authentication request 305 comprising the user identity to that server. The second AAA server 204 may send a corresponding authentication message 306 comprising the user identity and its own server ID (also called server name) to the HSS 202. However, the HSS 202 is not aware of the failure of the first AAA server 203 either. After comparing the received server ID and the server ID stored in its database originally associated with the user identity it may inform the second AAA server 204 in step 307 that the first AAA server 203 is associated with the user identity and thus should handle the request. Accordingly, the second AAA server 204 may reply to the affected network entity 201 in step 308 that the first AAA server 203 should be used instead. The affected network entity 201 may try to send the authentication message to the failed AAA server 203 again as depicted in step 303 or simply determines that authentication request 303 has failed in step 209 and, as the result, the session is not established.
The failure of an AAA server 203 may also happen after a communication session associated with the user identity has been established. Re-authentication message may be triggered in step 302 and the failure of the first AAA server 203 may cause the session to be lost in the similar way as shown in the above example.